/Engine/Plugins/Runtime/GeometryProcessing/Source/DynamicMesh/Public/Polygroups/PolygroupsGenerator.h
Find Polygroups by randomly picking initial seed triangles and then flood-filling outwards, stopping when the opening angle at an edge is larger than the angle defined by the DotTolerance.
bool FindPolygroupsFromFaceNormals ( double DotTolerance, bool bRespectUVSeams, bool bRespectNormalSeams )